Optical coherent communication systems widely researched due to their potential ability to communicate vast amounts of information quickly. In optical coherent communication systems, a so-called “optical hybrid” is used to overlay a received optical signal and a local oscillator optical signal, resulting in a demodulation of the received optical signal and yielding in-phase components I and I and quadrature components Q and Q. These components I, I, Q and Q, are then transformed into electrical signals using photodetectors. This configuration is sometimes called a “phase diversity receiver.” Electronic logic circuitry can then be used to compare the electrical signals to one another, one or more thresholds, or both, to yield output data.
The local oscillator optical signal and received optical signal should have the same polarization orientation to beat properly with each other. Unfortunately, by the time the received optical signal has reached the receiver, it has experienced arbitrary polarization transformation as a result of being transmitted over a fiber. Therefore, the receiver has either to track the polarization state using a polarization controller in the local oscillator optical signal path or use a configuration called “polarization diversity receiver.” A polarization diversity receiver is preferred if one wants to use a polarization multiplexed signal to use two, preferably linear and orthogonal, polarization states to transmit data. A conventional polarization diversity receiver employs a polarization splitter to split the signal path and two optical hybrids, each fed with a properly aligned local oscillator optical signal (see, e.g., Kazovsky, “Phase- and Polarization-Diversity Coherent Optical Techniques,” J. Lightwave Technol., vol. LT-7, no. 2, pp. 279-292, February 1989).
Unfortunately, an optical hybrid is a relatively expensive device. A polarization diversity receiver that employs two optical hybrids can be so expensive that many applications that could benefit from it cannot justify it simply as a result of the cost alone.
Accordingly, what is needed in the art is a better architecture for a polarization diversity receiver. More specifically, what is needed in the art is a polarization diversity receiver with a reduced manufacturing cost.